Stack management unit and method for a processor having a stack

ABSTRACT

The present invention provides a stack management unit including a stack cache to accelerate data transfers between the stack-based computing system and the stack. In one embodiment, the stack management unit includes a stack cache, a dribble manager unit, and a stack control unit. The dribble manager unit includes a fill control unit and a spill control unit. Since the vast majority of memory accesses to the stack occur at or near the top of the stack, the dribble manager unit maintains the top portion of the stack in the stack cache. Specifically, when the stack-based computing system is pushing data onto the stack and a spill condition occurs, the spill control unit transfers data from the bottom of the stack cache to the stack so that the top portion of the stack remains in the stack cache. When the stack-based computing system is popping data off of the stack and a fill condition occurs, the fill control unit transfer data from the stack to the bottom of the stack cache to maintain the top portion of the stack in the stack cache. Typically, a fill condition occurs as the stack cache becomes empty and a spill condition occurs as the stack cache becomes full.

RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.60/010,527, filed Jan. 24, 1996, entitled "Methods and Apparatuses forImplementing the JAVA Virtual Machine" (JAVA is a trademark of SunMicrosystems, Inc.) and naming Marc Tremblay, James Michael O'Connor,Robert Garner, and William N. Joy as inventors, and is acontinuation-in-part application of U.S. application Ser. No.08/642,253, filed May 2, 1996, abandoned, entitled "METHODS ANDAPPARATUSES FOR IMPLEMENTING OPERAND STACK CACHE AS A CIRCULAR BUFFER"and naming Marc Tremblay and James Michael O'Connor as inventors thatalso claimed the benefit of U.S. Provisional Application No. 60/010,527,filed Jan. 24, 1996, entitled "Methods and Apparatuses for Implementingthe JAVA Virtual Machine" and naming Marc Tremblay, James MichaelO'Connor, Robert Garner, and William N. Joy as inventors.

Reference to Appendix I

A portion of the disclosure of this patent document including AppendixI, The JAVA Virtual Machine Specification and Appendix A thereto,contains material which is subject to copyright protection. Thecopyright owner has no objection to the facsimile reproduction by anyoneof the patent document or the patent disclosure, as it appears in theU.S. Patent and Trademark Office patent files or records, but otherwisereserves all copyright rights whatsoever.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to computer systems and, inparticular, to caching of stack memory architectures.

2. Discussion of Related Art

Many individuals and organizations in the computer and communicationsindustries tout the Internet as the fastest growing market on theplanet. In the 1990s, the number of users of the Internet appears to begrowing exponentially with no end in sight. In June of 1995, anestimated 6,642,000 hosts were connected to the Internet; thisrepresented an increase from an estimated 4,852,000 hosts in January,1995. The number of hosts appears to be growing at around 750% per year.Among the hosts, there were approximately 120,000 networks and over27,000 web servers. The number of web servers appears to beapproximately doubling every 53 days.

In July 1995, with over 1,000,000 active Internet users, over 12,505usenet news groups, and over 10,000,000 usenet readers, the Internetappears to be destined to explode into a very large market for a widevariety of information and multimedia services.

In addition to the public carrier network or Internet, many corporationsand other businesses are shifting their internal information systemsonto an intranet as a way of more effectively sharing information withina corporate or private network. The basic infrastructure for an intranetis an internal network connecting servers and desktops, which may or maynot be connected to the Internet through a firewall. These intranetsprovide services to desktops via standard open network protocols whichare well established in the industry. Intranets provide many benefits tothe enterprises which employ them, such as simplified internalinformation management and improved internal communication using thebrowser paradigm. Integrating Internet technologies with a company'senterprise infrastructure and legacy systems also leverages existingtechnology investment for the party employing an intranet. As discussedabove, intranets and the Internet are closely related, with intranetsbeing used for internal and secure communications within the businessand the Internet being used for external transactions between thebusiness and the outside world. For the purposes of this document, theterm "networks" includes both the Internet and intranets. However, thedistinction between the Internet and an intranet should be born in mindwhere applicable.

In 1990, programmers at Sun Microsystems wrote a universal programminglanguage. This language was eventually named the JAVA programminglanguage. (JAVA is a trademark of Sun Microsystems of Mountain View,Calif.) The JAVA programming language resulted from programming effortswhich initially were intended to be coded in the C++ programminglanguage; therefore, the JAVA programming language has many commonaltieswith the C++ programming language. However, the JAVA programminglanguage is a simple, object-oriented, distributed, interpreted yet highperformance, robust yet safe, secure, dynamic, architecture neutral,portable, and multi-threaded language.

The JAVA programming language has emerged as the programming language ofchoice for the Internet as many large hardware and software companieshave licensed it from Sun Microsystems. The JAVA programming languageand environment is designed to solve a number of problems in modernprogramming practice. The JAVA programming language omits many rarelyused, poorly understood, and confusing features of the C++ programminglanguage. These omitted features primarily consist of operatoroverloading, multiple inheritance, and extensive automatic coercions.The JAVA programming language includes automatic garbage collection thatsimplifies the task of programming because it is no longer necessary toallocate and free memory as in the C programming language. The JAVAprogramming language restricts the use of pointers as defined in the Cprogramming language, and instead has true arrays in which array boundsare explicitly checked, thereby eliminating vulnerability to manyviruses and nasty bugs. The JAVA programming language includesobjective-C interfaces and specific exception handlers.

The JAVA programming language has an extensive library of routines forcoping easily with TCP/IP protocol (Transmission Control Protocol basedon Internet protocol), HTTP (Hypertext Transfer Protocol) and FTP (FileTransfer Protocol). The JAVA programming language is intended to be usedin networked/distributed environments. The JAVA programming languageenabled the construction of virus-free, tamper-free systems. Theauthentication techniques are based on public-key encryption.

Many stack-based computing systems, including those implementing theJAVA virtual machine, use relatively slow memory devices to store thestack. In general, adding a cache for slow memory devices increasesoverall memory performance only if the vast majority of memory requestsresult in cache hits, i.e. the requested memory address is within thecache. Conventional cache designs are designed for random access memoryarchitectures and do not perform well with stack-based memoryarchitectures. Therefore, a caching method and a caching apparatustargeted to improve stack-based memory architectures is desirable.

SUMMARY OF THE INVENTION

Accordingly, the present invention provides a stack management unitincluding a stack cache to accelerate data transfers between thestack-based computing system and the stack. In one embodiment, the stackmanagement unit includes a stack cache, a dribble manager unit, and astack control unit. Since the vast majority of memory accesses to thestack occur at or near the top of the stack, the dribble manager unitmaintains the top portion of the stack in the stack cache. Specifically,when the stack-based computing system is pushing data onto the stack andthe stack cache is almost full, the dribble manager unit transfers datafrom the bottom of the stack cache to the stack so that the top portionof the stack remains in the stack cache. When the stack-based computingsystem is popping data off of the stack and the stack cache is becomingempty, the dribble manager unit transfer data from the stack to thebottom of the stack cache to maintain the top portion of the stack inthe stack cache.

The stack cache includes a stack cache memory circuit, one or more readports, and one or more write ports. In one embodiment the stack cachememory circuit is a register file configured in a circular buffer memoryarchitecture. For the circular buffer architecture, the registers can beaddressed using modulo addressing. Typically, an OPTOP pointer is usedto define and point to the top memory location in the stack cache memorycircuit and a bottom pointer is used to define and point to the bottommemory location in the stack cache memory circuit. As data words arepushed or popped off of the stack, the OPTOP pointer is incremented ordecremented, respectively. Similarly, as data words are spilled orfilled between the stack cache memory circuit and the stack, the bottompointer is incremented or decremented, respectively.

Some embodiments of the stack management unit include a fill controlunit and a spill control unit. If the fill control unit detects a fillcondition, the fill control unit transfers data from the stack to thestack cache memory circuit. In one embodiment of the stack managementunit a fill condition occurs if an optop pointer is greater than a highwater mark. In another embodiment, a fill condition occurs if the numberof free memory locations in the stack cache memory circuit is greaterthan a low cache threshold or the number of used memory locations isless than the low cache threshold. Typically, the low water mark and thelow cache threshold are stored in programmable registers. The number offree memory locations can be determined with a modulo subtractor.

In one embodiment of the stack management unit a spill condition occursif the optop pointer is less than a low water mark. In anotherembodiment a spill condition occurs if the number of free locations inthe stack cache memory circuit is less than a high cache threshold orthe number of used memory locations is greater than the high cachethreshold. Typically, the low water mark and the low cache threshold arestored in programmable registers. The number of free memory locationscan be determined with a modulo subtractor.

For embodiments of the invention used with computing systems usingmethod calls, method frames are stored in two different memory circuits.The first memory circuit stores the execution environment of each methodcall, and the second memory circuit stores parameters, variables oroperands of the method calls. In one embodiment the executionenvironment includes a return program counter, a return frame, a returnconstant pool, a current method vector, and a current monitor address.In some embodiments, the memory circuits are stacks; therefore, thestack management unit described herein can be used to cache the memorycircuits.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a key to FIGS. 1A and 1B, which are a block diagram of oneembodiment of virtual machine hardware processor that utilizes the astack management unit of this invention.

FIG. 2 is a process flow diagram for generation of virtual machineinstructions that are used in one embodiment of this invention.

FIG. 3 illustrates an instruction pipeline implemented in the hardwareprocessor of FIG. 1.

FIG. 4A is an illustration of the one embodiment of the logicalorganization of a stack structure where each method frame includes alocal variable storage area, an environment storage area, and an operandstack utilized by the hardware processor of FIG. 1.

FIG. 4B is an illustration of an alternative embodiment of the logicalorganization of a stack structure where each method frame includes alocal variable storage area and an operand stack on the stack, and anenvironment storage area for the method frame is included on a separateexecution environment stack.

FIG. 4C is an illustration of an alternative embodiment of the stackmanagement unit for the stack and execution environment stack of FIG.4B.

FIG. 4D is an illustration of one embodiment of the local variableslook-aside cache in the stack management unit of FIG. 1.

FIG. 5 illustrates several possible add-ons to the hardware processor ofFIG. 1.

FIG. 6 illustrates a block diagram of one embodiment of a stack cachemanagement unit in accordance with this invention.

FIG. 7 illustrates the memory architecture of one embodiment of a stackcache in accordance with this invention.

FIG. 8 illustrates the contents of a register or memory location of oneembodiment of a stack cache in accordance with this invention.

FIG. 9 illustrates a block diagram of one embodiment of a dribblemanager unit in accordance with this invention.

FIG. 10A illustrates a block diagram of another embodiment of a dribblemanager unit in accordance with this invention.

FIG. 10B illustrates a block diagram of another embodiment of a dribblemanager unit in accordance with this invention.

FIG. 11 illustrates a block diagram of a portion of an embodiment of adribble manager unit in accordance with this invention.

FIG. 12 illustrates a pointer generation circuit for one embodiment of astack cache in accordance with this invention.

These and other features and advantages of the present invention will beapparent from the Figures as explained in the Detailed Description ofthe Invention. Like or similar features are designated by the samereference numeral(s) throughout the drawings and the DetailedDescription of the Invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates one embodiment of a virtual machine instructionhardware processor 100, hereinafter hardware processor 100, thatincludes a stack management unit in accordance with the presentinvention, and that directly executes virtual machine instructions thatare processor architecture independent. The performance of hardwareprocessor 100 in executing JAVA virtual machine instructions is muchbetter than high-end CPUs, such as the Intel PENTIUM microprocessor orthe Sun Microsystems ULTRASPARC processor, (ULTRASPARC is a trademark ofSun Microsystems of Mountain View, Calif., and PENTIUM is a trademark ofIntel Corp. of Sunnyvale, Calif.) interpreting the same virtual machineinstructions with a software JAVA interpreter. or with a JAVAjust-in-time compiler; is low cost; and exhibits low power consumption.As a result, hardware processor 100 is well suited for portableapplications. Hardware processor 100 provides similar advantages forother virtual machine stack-based architectures as well as for virtualmachines utilizing features such as garbage collection, threadsynchronization, etc.

In view of these characteristics, a system based on hardware processor100 presents attractive price for performance characteristics, if notthe best overall performance, as compared with alternative virtualmachine execution environments including software interpreters andjust-in-time compilers. Nonetheless, the present invention is notlimited to virtual machine hardware processor embodiments, andencompasses any suitable stack-based, or non-stack-based machineimplementations, including implementations emulating the JAVA virtualmachine as a software interpreter, compiling JAVA virtual machineinstructions (either in batch or just-in-time) to machine instructionnative to a particular hardware processor, or providing hardwareimplementing the JAVA virtual machine in microcode, directly in silicon,or in some combination thereof.

Regarding price for performance characteristics, hardware processor 100has the advantage that the 250 Kilobytes to 500 Kilobytes (Kbytes) ofmemory storage, e.g., read-only memory or random access memory,typically required by a software interpreter, is eliminated.

A simulation of hardware processor 100 showed that hardware processor100 executes virtual machine instructions twenty times faster than asoftware interpreter running on a variety of applications on a PENTIUMprocessor clocked at the same clock rate as hardware processor 100, andexecuting the same virtual machine instructions. Another simulation ofhardware processor 100 showed that hardware processor 100 executesvirtual machine instructions five times faster than a just-in-timecompiler running on a PENTIUM processor running at the same clock rateas hardware processor 100, and executing the same virtual machineinstructions.

In environments in which the expense of the memory required for asoftware virtual machine instruction interpreter is prohibitive,hardware processor 100 is advantageous. These applications include, forexample, an Internet chip for network appliances, a cellular telephoneprocessor, other telecommunications integrated circuits, or otherlow-power, low-cost applications such as embedded processors, andportable devices.

The present invention includes a stack management unit 150 that utilizesa stack cache 155 to accelerate data transfers for execution unit 140.Although, stack management unit 150 can be an integral part of hardwareprocessor 100 as shown in FIG. 1, many embodiments of stack managementunit 150 are not integrated with a hardware processor since stackmanagement in accordance with the present invention can be adapted foruse with any stack-based computing system. In one embodiment, stackmanagement unit 150 includes a stack cache 155, a dribble manager unit151, and a stack control unit 152. When hardware processor 100 ispushing data onto stack 400 (FIG. 4(a)) and stack cache 155 is almostfull, dribble manager unit 151 transfers data from the bottom of stackcache 155 to stack 400 through data cache unit 160, so that the topportion of stack 400 remains in stack cache 155. When hardware processor100 is popping data off of stack 400 and stack cache 155 is almostempty, dribble manager unit 151 transfers data from stack 400 to thebottom of stack cache 155 so that the top portion of stack 400 ismaintained in stack cache 155.

As used herein, a virtual machine is an abstract computing machine that,like a real computing machine, has an instruction set and uses variousmemory areas. A virtual machine specification defines a set of processorarchitecture independent virtual machine instructions that are executedby a virtual machine implementation, e.g., hardware processor 100. Eachvirtual machine instruction defines a specific operation that is to beperformed. The virtual computing machine need not understand thecomputer language that is used to generate virtual machine instructionsor the underlying implementation of the virtual machine. Only aparticular file format for virtual machine instructions needs to beunderstood.

In an exemplary embodiment, the virtual machine instructions are JAVAvirtual machine instructions. Each JAVA virtual machine instructionincludes one or more bytes that encode instruction identifyinginformation, operands, and any other required information. Appendix I,which is incorporated herein by reference in its entirety, includes anillustrative set of the JAVA virtual machine instructions. Theparticular set of virtual machine instructions utilized is not anessential aspect of this invention. In view of the virtual machineinstructions in Appendix I and this disclosure, those of skill in theart can modify the invention for a particular set of virtual machineinstructions, or for changes to the JAVA virtual machine specification.

A JAVA compiler JAVAC, (FIG. 2) that is executing on a computerplatform, converts an application 201 written in the JAVA computerlanguage to an architecture neutral object file format encoding acompiled instruction sequence 203, according to the JAVA Virtual MachineSpecification, that includes a compiled instruction set. However, forthis invention, only a source of virtual machine instructions andrelated information is needed. The method or technique used to generatethe source of virtual machine instructions and related information isnot essential to this invention.

Compiled instruction sequence 203 is executable on hardware processor100 as well as on any computer platform that implements the JAVA virtualmachine using, for example, a software interpreter or just-in-timecompiler. However, as described above, hardware processor 100 providessignificant performance advantages over the software implementations.

In this embodiment, hardware processor 100 (FIG. 1) processes the JAVAvirtual machine instructions, which include bytecodes. Hardwareprocessor 100, as explained more completely below, executes directlymost of the bytecodes. However, execution of some of the bytecodes isimplemented via microcode.

One strategy for selecting virtual machine instructions that areexecuted directly by hardware processor 100 is described herein by wayof an example. Thirty percent of the JAVA virtual machine instructionsare pure hardware translations; instructions implemented in this mannerinclude constant loading and simple stack operations. The next 50% ofthe virtual machine instructions are implemented mostly, but notentirely, in hardware and require some firmware assistance; theseinclude stack based operations and array instructions. The next 10% ofthe JAVA virtual machine instructions are implemented in hardware, butrequire significant firmware support as well; these include functioninvocation and function return. The remaining 10% of the JAVA virtualmachine instructions are not supported in hardware, but rather aresupported by a firmware trap and/or microcode; these include functionssuch as exception handlers. Herein, firmware means microcode stored inROM that when executed controls the operations of hardware processor100.

In one embodiment, hardware processor 100 includes an I/O bus and memoryinterface unit 110, an instruction cache unit 120 including instructioncache 125, an instruction decode unit 130, a unified execution unit 140,a stack management unit 150 including stack cache 155, a data cache unit160 including a data cache 165, and program counter and trap controllogic 170. Each of these units is described more completely below.

Also, as illustrated in FIG. 1, each unit includes several elements. Forclarity and to avoid distracting from the invention, theinterconnections between elements within a unit are not shown in FIG. 1.However, in view of the following description, those of skill in the artwill understand the interconnections and cooperation between theelements in a unit and between the various units.

The pipeline stages implemented using the units illustrated in FIG. 1include fetch, decode, execute, and write-back stages. If desired, extrastages for memory access or exception resolution are provided inhardware processor 100. FIG. 3 is an illustration of a four stagepipeline for execution of instructions in the exemplary embodiment ofprocessor 100. In fetch stage 301, a virtual machine instruction isfetched and placed in instruction buffer 124 (FIG. 1). The virtualmachine instruction is fetched from one of (i) a fixed size cache linefrom instruction cache 125 or (ii) external memory.

With regard to fetching, aside from instructions tableswitch andlookupswitch, (See Appendix I.) each virtual machine instruction isbetween one and five bytes long. Thus, to keep things simple, at leastforty bits are required to guarantee that all of a given instruction iscontained in the fetch.

Another alternative is to always fetch a predetermined number of bytes,for example, four bytes, starting with the opcode. This is sufficientfor 95% of JAVA virtual machine instructions (See Appendix I). For aninstruction requiring more than three bytes of operands, another cyclein the front end must be tolerated if four bytes are fetched. In thiscase, the instruction execution can be started with the first operandsfetched even if the full set of operands is not yet available.

In decode stage 302 (FIG. 3), the virtual machine instruction at thefront of instruction buffer 124 (FIG. 1) is decoded and instructionfolding is performed if possible. Stack cache 155 is accessed only ifneeded by the virtual machine instruction.

Register OPTOP, that contains a pointer OPTOP to a top of a stack 400(FIGS. 4A and 4B), is also updated in decode stage 302 (FIG. 3).

Herein, for convenience, the value in a register and the register areassigned the same reference numeral. Further, in the followingdiscussion, use of a register to store a pointer is illustrative only ofone embodiment. Depending on the specific implementation of theinvention, the pointer may be implemented using a hardware register, ahardware counter, a software counter, a software pointer, or otherequivalent embodiments known to those of skill in the art. Theparticular implementation selected is not essential to the invention,and typically is made based on a price to performance trade-off.

In execute stage 303, the virtual machine instruction is executed forone or more cycles. Typically, in execute stage 303, an ALU in integerunit 142 (FIG. 1) is used either to do an arithmetic computation or tocalculate the address of a load or a store from data cache unit (DCU)160. If necessary, traps are prioritized and taken at the end of executestage 303 (FIG. 3). For control flow instructions, the branch address iscalculated in execute stage 303, as well as the condition upon which thebranch is dependent.

Cache stage 304 is a non-pipelined stage. Data cache 165 (FIG. 1) isaccessed if needed during execution stage 303 (FIG. 3). The reason thatstage 304 is non-pipelined is because hardware processor 100 is astack-based machine. Thus, the instruction following a load is almostalways dependent on the value returned by the load. Consequently, inthis embodiment, the pipeline is held for one cycle for a data cacheaccess. This reduces the pipeline stages, and the die area taken by thepipeline for the extra registers and bypasses.

Write-back stage 305 is the last stage in the pipeline. In stage 305,the calculated data is written back to stack cache 155.

Hardware processor 100, in this embodiment, directly implements a stack400 (FIG. 4A) that supports the JAVA virtual machine stack-basedarchitecture (See Appendix I). Sixty-four entries on stack 400 arecontained on stack cache 155 in stack management unit 150. Some entriesin stack 400 may be duplicated on stack cache 155. Operations on dataare performed through stack cache 155.

Stack 400 of hardware processor 100 is primarily used as a repository ofinformation for methods. At any point in time, hardware processor 100 isexecuting a single method. Each method has memory space, i.e., a methodframe on stack 400, allocated for a set of local variables, an operandstack, and an execution environment structure.

A new method frame, e.g., method frame two 410, is allocated by hardwareprocessor 100 upon a method invocation in execution stage 303 (FIG. 3)and becomes the current frame, i.e., the frame of the current method.Current frame 410 (FIG. 4A), as well as the other method frames, maycontain a part of or all of the following six entities, depending onvarious method invoking situations:

Object reference;

Incoming arguments;

Local variables;

Invoker's method context;

Operand stack; and

Return value from method.

In FIG. 4A, object reference, incoming arguments, and local variablesare included in arguments and local variables area 421. The invoker'smethod context is included in execution environment 422, sometimescalled frame state, that in turn includes: a return program countervalue 431 that is the address of the virtual machine instruction, e.g.,JAVA opcode, next to the method invoke instruction; a return frame 432that is the location of the calling method's frame; a return constantpool pointer 433 that is a pointer to the calling method's constant pooltable; a current method vector 434 that is the base address of thecurrent method's vector table; and a current monitor address 435 that isthe address of the current method's monitor.

The object reference is an indirect pointer to an object-storagerepresenting the object being targeted for the method invocation. JAVAcompiler JAVAC (See FIG. 2.) generates an instruction to push thispointer onto operand stack 423 prior to generating an invokeinstruction. This object reference is accessible as local variable zeroduring the execution of the method. This indirect pointer is notavailable for a static method invocation as there is no target-objectdefined for a static method invocation.

The list of incoming arguments transfers information from the callingmethod to the invoked method. Like the object reference, the incomingarguments are pushed onto stack 400 by JAVA compiler generatedinstructions and may be accessed as local variables. JAVA compiler JAVAC(See FIG. 2.) statically generates a list of arguments for currentmethod 410 (FIG. 4A), and hardware processor 100 determines the numberof arguments from the list. When the object reference is present in theframe for a non-static method invocation, the first argument isaccessible as local variable one. For a static method invocation, thefirst argument becomes local variable zero.

For 64-bit arguments, as well as 64-bit entities in general, the upper32-bits, i.e., the 32 most significant bits, of a 64-bit entity areplaced on the upper location of stack 400, i.e., pushed on the stacklast. For example, when a 64-bit entity is on the top of stack 400, theupper 32-bit portion of the 64-bit entity is on the top of the stack,and the lower 32-bit portion of the 64-bit entity is in the storagelocation immediately adjacent to the top of stack 400.

The local variable area on stack 400 (FIG. 4A) for current method 410represents temporary variable storage space which is allocated andremains effective during invocation of method 410. JAVA compiler JAVAC(FIG. 2) statically determines the required number of local variablesand hardware processor 100 allocates temporary variable storage spaceaccordingly.

When a method is executing on hardware processor 100, the localvariables typically reside in stack cache 155 and are addressed asoffsets from pointer VARS (FIGS. 1 and 4A), which points to the positionof the local variable zero. Instructions are provided to load the valuesof local variables onto operand stack 423 and store values from operandstack into local variables area 421.

The information in execution environment 422 includes the invoker'smethod context. When a new frame is built for the current method,hardware processor 100 pushes the invoker's method context onto newlyallocated frame 410, and later utilizes the information to restore theinvoker's method context before returning. Pointer FRAME (FIGS. 1 and4A) is a pointer to the execution environment of the current method. Inthe exemplary embodiment, each register in register set 144 (FIG. 1) is32-bits wide.

Operand stack 423 is allocated to support the execution of the virtualmachine instructions within the current method. Program counter registerPC (FIG. 1) contains the address of the next instruction, e.g., opcode,to be executed. Locations on operand stack 423 (FIG. 4A) are used tostore the operands of virtual machine instructions, providing bothsource and target storage locations for instruction execution. The sizeof operand stack 423 is statically determined by JAVA compiler JAVAC(FIG. 2) and hardware processor 100 allocates space for operand stack423 accordingly. Register OPTOP (FIGS. 1 and 4A) holds a pointer to atop of operand stack 423.

The invoked method may return its execution result onto the invoker'stop of stack, so that the invoker can access the return value withoperand stack references. The return value is placed on the area wherean object reference or an argument is pushed before a method invocation.

Simulation results on the JAVA virtual machine indicate that methodinvocation consumes a significant portion of the execution time(20-40%). Given this attractive target for accelerating execution ofvirtual machine instructions, hardware support for method invocation isincluded in hardware processor 100, as described more completely below.

The beginning of the stack frame of a newly invoked method, i.e., theobject reference and the arguments passed by the caller, are alreadystored on stack 400 since the object reference and the incomingarguments come from the top of the stack of the caller. As explainedabove, following these items on stack 400, the local variables areloaded and then the execution environment is loaded.

One way to speed up this process is for hardware processor 100 to loadthe execution environment in the background and indicate what has beenloaded so far, e.g., simple one bit scoreboarding. Hardware processor100 tries to execute the bytecodes of the called method as soon aspossible, even though stack 400 is not completely loaded. If accessesare made to variables already loaded, overlapping of execution withloading of stack 400 is achieved, otherwise a hardware interlock occursand hardware processor 100 just waits for the variable or variables inthe execution environment to be loaded.

FIG. 4B illustrates another way to accelerate method invocation. Insteadof storing the entire method frame in stack 400, the executionenvironment of each method frame is stored separately from the localvariable area and the operand stack of the method frame. Thus, in thisembodiment, stack 400B contains modified method frames, e.g., modifiedmethod frame 410B having only local variable area 421 and operand stack423. Execution environment 422 of the method frame is stored in anexecution environment memory 440. Storing the execution environment inexecution environment memory 440 reduces the amount of data in stackcache 155. Therefore, the size of stack cache 155 can be reduced.Furthermore, execution environment memory 440 and stack cache 155 can beaccessed simultaneously. Thus, method invocation can be accelerated byloading or storing the execution environment in parallel with loading orstoring data onto stack 400B.

In one embodiment of stack management unit 150, the memory architectureof execution environment memory 440 is also a stack. As modified methodframes are pushed onto stack 400B through stack cache 155, correspondingexecution environments are pushed onto execution environment memory 440.For example, since modified method frames 0 to 2, as shown in FIG. 42,are in stack 400B, execution environments (EE) 0 to 2, respectively, arestored in execution environment memory circuit 440.

To further enhance method invocation, an execution environment cache canbe added to improve the speed of saving and retrieving the executionenvironment during method invocation. The architecture described morecompletely below for stack cache 155, dribbler manager unit 151, andstack control unit 152 for caching stack 400, can also be applied tocaching execution environment memory 440.

FIG. 4C illustrates an embodiment of stack management unit 150 modifiedto support both stack 400B and execution environment memory 440.Specifically, the embodiment of stack management unit 150 in FIG. 4Cadds an execution environment stack cache 450, an execution environmentdribble manager unit 460, and an execution environment stack controlunit 470. Typically, execution dribble manager unit 460 transfers anentire execution environment between execution environment cache 450 andexecution environment memory 440 during a spill operation or a filloperation.

I/O Bus and Memory Interface Unit

I/O bus and memory interface unit 110 (FIG. 1), sometimes calledinterface unit 110, implements an interface between hardware processor100 and a memory hierarchy which in an exemplary embodiment includesexternal memory and may optionally include memory storage and/orinterfaces on the same die as hardware processor 100. In thisembodiment, I/O controller 111 interfaces with external I/O devices andmemory controller 112 interfaces with external memory. Herein, externalmemory means memory external to hardware processor 100. However,external memory either may be included on the same die as hardwareprocessor 100, may be external to the die containing hardware processor100, or may include both on- and off-die portions.

In another embodiment, requests to I/O devices go through memorycontroller 112 which maintains an address map of the entire systemincluding hardware processor 100. On the memory bus of this embodiment,hardware processor 100 is the only master and does not have to arbitrateto use the memory bus.

Hence, alternatives for the input/output bus that interfaces with I/Obus and memory interface unit 110 include supporting memory-mappedschemes, providing direct support for PCI, PCMCIA, or other standardbusses. Fast graphics (w/ VIS or other technology) may optionally beincluded on the die with hardware processor 100.

I/O bus and memory interface unit 110 generates read and write requeststo external memory. Specifically, interface unit 110 provides aninterface for instruction cache and data cache controllers 121 and 161to the external memory. Interface unit 110 includes arbitration logicfor internal requests from instruction cache controller 121 and datacache controller 161 to access external memory and in response to arequest initiates either a read or a write request on the memory bus tothe external memory. A request from data cache controller 161 is alwaystreated as higher priority relative to a request from instruction cachecontroller 161.

Interface unit 110 provides an acknowledgment signal to the requestinginstruction cache controller 121, or data cache controller 161 on readcycles so that the requesting controller can latch the data. On writecycles, the acknowledgment signal from interface unit 110 is used forflow control so that the requesting instruction cache controller 121 ordata cache controller 161 does not generate a new request when there isone pending. Interface unit 110 also handles errors generated on thememory bus to the external memory.

Instruction Cache Unit

Instruction cache unit (ICU) 120 (FIG. 1) fetches virtual machineinstructions from instruction cache 125 and provides the instructions toinstruction decode unit 130. In this embodiment, upon a instructioncache hit, instruction cache controller 121, in one cycle, transfers aninstruction from instruction cache 125 to instruction buffer 124 wherethe instruction is held until integer execution unit IEU, that isdescribed more completely below, is ready to process the instruction.This separates the rest of pipeline 300 (FIG. 3) in hardware processor100 from fetch stage 301. If it is undesirable to incur the complexityof supporting an instruction-buffer type of arrangement, a temporary oneinstruction register is sufficient for most purposes. However,instruction fetching, caching, and buffering should provide sufficientinstruction bandwidth to support instruction folding as described below.

The front end of hardware processor 100 is largely separate from therest of hardware processor 100. Ideally, one instruction per cycle isdelivered to the execution pipeline.

The instructions are aligned on an arbitrary eight-bit boundary by bytealigner circuit 122 in response to a signal from instruction decode unit130. Thus, the front end of hardware processor 100 efficiently dealswith fetching from any byte position. Also, hardware processor 100 dealswith the problems of instructions that span multiple cache lines ofcache 125. In this case, since the opcode is the first byte, the designis able to tolerate an extra cycle of fetch latency for the operands.Thus, a very simple de-coupling between the fetching and execution ofthe bytecodes is possible.

In case of an instruction cache miss, instruction cache controller 121generates an external memory request for the missed instruction to I/Obus and memory interface unit 110. If instruction buffer 124 is empty,or nearly empty, when there is an instruction cache miss, instructiondecode unit 130 is stalled, i.e., pipeline 300 is stalled. Specifically,instruction cache controller 121 generates a stall signal upon a cachemiss which is used along with an instruction buffer empty signal todetermine whether to stall pipeline 300. Instruction cache 125 can beinvalidated to accommodate self-modifying code, e.g., instruction cachecontroller 121 can invalidate a particular line in instruction cache125.

Thus, instruction cache controller 121 determines the next instructionto be fetched, i.e., which instruction in instruction cache 125 needs toaccessed, and generates address, data and control signals for data andtag RAMs in instruction cache 125. On a cache hit, four bytes of dataare fetched from instruction cache 125 in a single cycle, and a maximumof four bytes can be written into instruction buffer 124.

Byte aligner circuit 122 aligns the data out of the instruction cacheRAM and feeds the aligned data to instruction buffer 124. As explainedmore completely below, the first two bytes in instruction buffer 124 aredecoded to determine the length of the virtual machine instruction.Instruction buffer 124 tracks the valid instructions in the queue andupdates the entries, as explained more completely below.

Instruction cache controller 121 also provides the data path and controlfor handling instruction cache misses. On an instruction cache miss,instruction cache controller 121 generates a cache fill request to I/Obus and memory interface unit 110.

On receiving data from external memory, instruction cache controller 121writes the data into instruction cache 125 and the data are alsobypassed into instruction buffer 124. Data are bypassed to instructionbuffer 124 as soon as the data are available from external memory, andbefore the completion of the cache fill.

Instruction cache controller 121 continues fetching sequential datauntil instruction buffer 124 is full or a branch or trap has takenplace. In one embodiment, instruction buffer 124 is considered full ifthere are more than eight bytes of valid entries in buffer 124. Thus,typically, eight bytes of data are written into instruction cache 125from external memory in response to the cache fill request sent tointerface unit 110 by instruction cache unit 120. If there is a branchor trap taken while processing an instruction cache miss, only after thecompletion of the miss processing is the trap or branch executed.

When an error is generated during an instruction cache fill transaction,a fault indication is generated and stored into instruction buffer 124along with the virtual machine instruction, i.e., a fault bit is set.The line is not written into instruction cache 125. Thus, the erroneouscache fill transaction acts like a non-cacheable transaction except thata fault bit is set. When the instruction is decoded, a trap is taken.

Instruction cache controller 121 also services non-cacheable instructionreads. An instruction cache enable (ICE) bit, in a processor statusregister in register set 144, is used to define whether a load can becached. If the instruction cache enable bit is cleared, instructioncache unit 120 treats all loads as non-cacheable loads. Instructioncache controller 121 issues a non-cacheable request to interface unit110 for non-cacheable instructions. When the data are available on acache fill bus for the non-cacheable instruction, the data are bypassedinto instruction buffer 124 and are not written into instruction cache125.

In this embodiment, instruction cache 125 is a direct-mapped, eight-byteline size cache. Instruction cache 125 has a single cycle latency. Thecache size is configurable to 0K, 1K, 2K, 4K, 8K and 16K byte sizeswhere K means kilo. The default size is 4K bytes. Each line has a cachetag entry associated with the line. Each cache tag contains a twenty bitaddress tag field and one valid bit for the default 4K byte size.

Instruction buffer 124, which, in an exemplary embodiment, is atwelve-byte deep first-in, first-out (FIFO) buffer, de-links fetch stage301 (FIG. 3) from the rest of pipeline 300 for performance reasons. Eachinstruction in buffer 124 (FIG. 1) has an associated valid bit and anerror bit. When the valid bit is set, the instruction associated withthat valid bit is a valid instruction. When the error bit is set, thefetch of the instruction associated with that error bit was an erroneoustransaction. Instruction buffer 124 includes an instruction buffercontrol circuit (not shown) that generates signals to pass data to andfrom instruction buffer 124 and that keeps track of the valid entries ininstruction buffer 124, i.e., those with valid bits set.

In an exemplary embodiment, four bytes can be received into instructionbuffer 124 in a given cycle. Up to five bytes, representing up to twovirtual machine instructions, can be read out of instruction buffer 124in a given cycle. Alternative embodiments, particularly those providingfolding of multi-byte virtual machine instructions and/or thoseproviding folding of more than two virtual machine instructions, providehigher input and output bandwidth. Persons of ordinary skill in the artwill recognize a variety of suitable instruction buffer designsincluding, for example, alignment logic, circular buffer design, etc.When a branch or trap is taken, all the entries in instruction buffer124 are nullified and the branch/trap data moves to the top ofinstruction buffer 124.

In the embodiment of FIG. 1, a unified execution unit 140 is shown.However, in another embodiment, instruction decode unit 130, integerunit 142, and stack management unit 150 are considered a single integerexecution unit, and floating point execution unit 143 is a separateoptional unit. In still other embodiments, the various elements in theexecution unit may be implemented using the execution unit of anotherprocessor. In general the various elements included in the various unitsof FIG. 1 are exemplary only of one embodiment. Each unit could beimplemented with all or some of the elements shown. Again, the decisionis largely dependent upon a price vs. performance trade-off.

Instruction Decode Unit

As explained above, virtual machine instructions are decoded in decodestage 302 (FIG. 3) of pipeline 300. In an exemplary embodiment, twobytes, that can correspond to two virtual machine instructions, arefetched from instruction buffer 124 (FIG. 1). The two bytes are decodedin parallel to determine if the two bytes correspond to two virtualmachine instructions, e.g., a first load top of stack instruction and asecond add top two stack entries instruction, that can be folded into asingle equivalent operation. Folding refers to supplying a singleequivalent operation corresponding to two or more virtual machineinstructions.

In an exemplary hardware processor 100 embodiment, a single-byte firstinstruction can be folded with a second instruction. However,alternative embodiments provide folding of more than two virtual machineinstructions, e.g., two to four virtual machine instructions, and ofmulti-byte virtual machine instructions, though at the cost ofinstruction decoder complexity and increased instruction bandwidth. SeeU.S. patent application Ser. No. 08/786,351, entitled "INSTRUCTIONFOLDING FOR A STACK-BASED MACHINE" naming Marc Tremblay and JamesMichael O'Connor as inventors, assigned to the assignee of thisapplication, and filed on even date herewith which is incorporatedherein by reference in its entirety. In the exemplary processor 100embodiment, if the first byte, which corresponds to the first virtualmachine instruction, is a multi-byte instruction, the first and secondinstructions are not folded.

An optional current object loader folder 132 exploits instructionfolding, such as that described above, and in greater detail in U.S.patent application Ser. No. 08/786,351, entitled "INSTRUCTION FOLDINGFOR A STACK-BASED MACHINE" naming Marc Tremblay and James MichaelO'Connor as inventors, assigned to the assignee of this application, andfiled on even date herewith with, which is incorporated herein byreference in its entirety, in virtual machine instruction sequenceswhich simulation results have shown to be particularly frequent andtherefore a desirable target for optimization. In particular, a methodinvocation typically loads an object reference for the correspondingobject onto the operand stack and fetches a field from the object.Instruction folding allow this extremely common virtual machineinstruction sequence to be executed using an equivalent foldedoperation.

Quick variants are not part of the virtual machine instruction set (SeeChapter 3 of Appendix I), and are invisible outside of a JAVA virtualmachine implementation. However, inside a virtual machineimplementation, quick variants have proven to be an effectiveoptimization. (See Appendix A in Appendix I; which is an integral partof this specification.) Supporting writes for updates of variousinstructions to quick variants in a non-quick to quick translator cache131 changes the normal virtual machine instruction to a quick virtualmachine instruction to take advantage of the large benefits bought fromthe quick variants. In particular, as described in more detail in U.S.patent application Ser. No. 08/788,805, entitled "NON-QUICK INSTRUCTIONACCELERATOR AND METHOD OF IMPLEMENTING SAME" naming Marc Tremblay andJames Michael O'Connor as inventors, assigned to the assignee of thisapplication, and filed on even date herewith with, which is incorporatedherein by reference in its entirety, when the information required toinitiate execution of an instruction has been assembled for the firsttime, the information is stored in a cache along with the value ofprogram counter PC as tag in non-quick to quick translator cache 131 andthe instruction is identified as a quick-variant. In one embodiment,this is done with self-modifying code.

Upon a subsequent call of that instruction, instruction decode unit 130detects that the instruction is identified as a quick-variant and simplyretrieves the information needed to initiate execution of theinstruction from non-quick to quick translator cache 131. Non-quick toquick translator cache is an optional feature of hardware processor 100.

With regard to branching, a very short pipe with quick branch resolutionis sufficient for most implementations. However, an appropriate simplebranch prediction mechanism can alternatively be introduced, e.g.,branch predictor circuit 133. Implementations for branch predictorcircuit 133 include branching based on opcode, branching based onoffset, or branching based on a two-bit counter mechanism.

The JAVA virtual machine specification defines an instructioninvokenonvirtual, opcode 183, which, upon execution, invokes methods.The opcode is followed by an index byte one and an index byte two. (SeeAppendix I.) Operand stack 423 contains a reference to an object andsome number of arguments when this instruction is executed.

Index bytes one and two are used to generate an index into the constantpool of the current class. The item in the constant pool at that indexpoints to a complete method signature and class. Signatures are definedin Appendix I and that description is incorporated herein by reference.

The method signature, a short, unique identifier for each method, islooked up in a method table of the class indicated. The result of thelookup is a method block that indicates the type of method and thenumber of arguments for the method. The object reference and argumentsare popped off this method's stack and become initial values of thelocal variables of the new method. The execution then resumes with thefirst instruction of the new method. Upon execution, instructionsinvokevirtual, opcode 182, and invokestatic, opcode 184, invokeprocesses similar to that just described. In each case, a pointer isused to lookup a method block.

A method argument cache 134, that also is an optional feature ofhardware processor 100, is used, in a first embodiment, to store themethod block of a method for use after the first call to the method,along with the pointer to the method block as a tag. Instruction decodeunit 130 uses index bytes one and two to generate the pointer and thenuses the pointer to retrieve the method block for that pointer in cache134. This permits building the stack frame for the newly invoked methodmore rapidly in the background in subsequent invocations of the method.Alternative embodiments may use a program counter or method identifieras a reference into cache 134. If there is a cache miss, the instructionis executed in the normal fashion and cache 134 is updated accordingly.The particular process used to determine which cache entry isoverwritten is not an essential aspect of this invention. Aleast-recently used criterion could be implemented, for example.

In an alternative embodiment, method argument cache 134 is used to storethe pointer to the method block, for use after the first call to themethod, along with the value of program counter PC of the method as atag. Instruction decode unit 130 uses the value of program counter PC toaccess cache 134. If the value of program counter PC is equal to one ofthe tags in cache 134, cache 134 supplies the pointer stored with thattag to instruction decode unit 130. Instruction decode unit 130 uses thesupplied pointer to retrieve the method block for the method. In view ofthese two embodiments, other alternative embodiments will be apparent tothose of skill in the art.

Wide index forwarder 136, which is an optional element of hardwareprocessor 100, is a specific embodiment of instruction folding forinstruction wide. Wide index forwarder 136 handles an opcode encoding anextension of an index operand for an immediately subsequent virtualmachine instruction. In this way, wide index forwarder 136 allowsinstruction decode unit 130 to provide indices into local variablestorage 421 when the number of local variables exceeds that addressablewith a single byte index without incurring a separate execution cyclefor instruction wide.

Aspects of instruction decoder 135, particularly instruction folding,non-quick to quick translator cache 131, current object loader folder132, branch predictor 133, method argument cache 134, and wide indexforwarder 136 are also useful in implementations that utilize a softwareinterpreter or just-in-time compiler, since these elements can be usedto accelerate the operation of the software interpreter or just-in-timecompiler. In such an implementation, typically, the virtual machineinstructions are translated to an instruction for the processorexecuting the interpreter or compiler, e.g., any one of a Sun processor,a DEC processor, an Intel processor, or a Motorola processor, forexample, and the operation of the elements is modified to supportexecution on that processor. The translation from the virtual machineinstruction to the other processor instruction can be done either with atranslator in a ROM or a simple software translator. For additionalexamples of dual instruction set processors, see U.S. patent applicationSer. No. 08/787,618, entitled "A PROCESSOR FOR EXECUTING INSTRUCTIONSETS RECEIVED FROM A NETWORK OR FROM A LOCAL MEMORY" naming MarcTremblay and James Michael O'Connor as inventors, assigned to theassignee of this application, and filed on even date herewith, which isincorporated herein by reference in its entirety

Integer Execution Unit

Integer execution unit IEU, that includes instruction decode unit 130,integer unit 142, and stack management unit 150, is responsible for theexecution of all the virtual machine instructions except the floatingpoint related instructions. The floating point related instructions areexecuted in floating point unit 143.

Integer execution unit IEU interacts at the front end with instructionscache unit 120 to fetch instructions, with floating point unit (FPU) 143to execute floating point instructions, and finally with data cache unit(DCU) 160 to execute load and store related instructions. Integerexecution unit IEU also contains microcode ROM 141 which containsinstructions to execute certain virtual machine instructions associatedwith integer operations.

Integer execution unit IEU includes a cached portion of stack 400, i.e.,stack cache 155. Stack cache 155 provides fast storage for operand stackand local variable entries associated with a current method, e.g.,operand stack 423 and local variable storage 421 entries. Although,stack cache 155 may provide sufficient storage for all operand stack andlocal variable entries associated with a current method, depending onthe number of operand stack and local variable entries, less than all oflocal variable entries or less than all of both local variable entriesand operand stack entries may be represented in stack cache 155.Similarly, additional entries, e.g., operand stack and or local variableentries for a calling method, may be represented in stack cache 155 ifspace allows.

Stack cache 155 is a sixty-four entry thirty-two-bit wide array ofregisters that is physically implemented as a register file in oneembodiment. Stack cache 155 has three read ports, two of which arededicated to integer execution unit IEU and one to dribble manager unit151. Stack cache 155 also has two write ports, one dedicated to integerexecution unit IEU and one to dribble manager unit 151.

Integer unit 142 maintains the various pointers which are used to accessvariables, such as local variables, and operand stack values, in stackcache 155. Integer unit 142 also maintains pointers to detect whether astack cache hit has taken place. Runtime exceptions are caught and dealtwith by exception handlers that are implemented using information inmicrocode ROM 141 and circuit 170.

Integer unit 142 contains a 32-bit ALU to support arithmetic operations.The operations supported by the ALU include: add, subtract, shift, and,or, exclusive or, compare, greater than, less than, and bypass. The ALUis also used to determine the address of conditional branches while aseparate comparator determines the outcome of the branch instruction.

The most common set of instructions which executes cleanly through thepipeline is the group of ALU instructions. The ALU instructions read theoperands from the top of stack 400 in decode stage 302 and use the ALUin execution stage 303 to compute the result. The result is written backto stack 400 in write-back stage 305. There are two levels of bypasswhich may be needed if consecutive ALU operations are accessing stackcache 155.

Since the stack cache ports are 32-bits wide in this embodiment, doubleprecision and long data operations take two cycles. A shifter is alsopresent as part of the ALU. If the operands are not available for theinstruction in decode stage 302, or at a maximum at the beginning ofexecution stage 303, an interlock holds the pipeline stages beforeexecution stage 303.

The instruction cache unit interface of integer execution unit IEU is avalid/accept interface, where instruction cache unit 120 deliversinstructions to instruction decode unit 130 in fixed fields along withvalid bits. Instruction decoder 135 responds by signaling how much bytealigner circuit 122 needs to shift, or how many bytes instruction decodeunit 130 could consume in decode stage 302. The instruction cache unitinterface also signals to instruction cache unit 120 the branchmis-predict condition, and the branch address in execution stage 303.Traps, when taken, are also similarly indicated to instruction cacheunit 120. Instruction cache unit 120 can hold integer unit 142 by notasserting any of the valid bits to instruction decode unit 130.Instruction decode unit 130 can hold instruction cache unit 120 by notasserting the shift signal to byte aligner circuit 122.

The data cache interface of integer execution unit IEU also is avalid-accept interface, where integer unit 142 signals, in executionstage 303, a load or store operation along with its attributes, e.g.,non-cached, special stores etc., to data cache controller 161 in datacache unit 160. Data cache unit 160 can return the data on a load, andcontrol integer unit 142 using a data control unit hold signal. On adata cache hit, data cache unit 160 returns the requested data, and thenreleases the pipeline.

On store operations, integer unit 142 also supplies the data along withthe address in execution stage 303. Data cache unit 160 can hold thepipeline in cache stage 304 if data cache unit 160 is busy, e.g., doinga line fill etc.

Floating point operations are dealt with specially by integer executionunit IEU. Instruction decoder 135 fetches and decodes floating pointunit 143 related instructions. Instruction decoder 135 sends thefloating point operation operands for execution to floating point unit142 in decode state 302. While floating point unit 143 is busy executingthe floating point operation, integer unit 142 halts the pipeline andwaits until floating point unit 143 signals to integer unit 142 that theresult is available.

A floating point ready signal from floating point unit 143 indicatesthat execution stage 303 of the floating point operation has concluded.In response to the floating point ready signal, the result is writtenback into stack cache 155 by integer unit 142. Floating point load andstores are entirely handled by integer execution unit IEU, since theoperands for both floating point unit 143 and integer unit 142 are foundin stack cache 155.

Stack Management Unit

A stack management unit 150 stores information, and provides operands toexecution unit 140. Stack management unit 150 also takes care ofoverflow and underflow conditions of stack cache 155.

In one embodiment, stack management unit 150 includes stack cache 155that, as described above, is a three read port, two write port registerfile in one embodiment; a stack control unit 152 which provides thenecessary control signals for two read ports and one write port that areused to retrieve operands for execution unit 140 and for storing databack from a write-back register or data cache 165 into stack cache 155;and a dribble manager 151 which speculatively dribbles data in and outof stack cache 155 into memory whenever there is an overflow orunderflow in stack cache 155. In the exemplary embodiment of FIG. 1,memory includes data cache 165 and any memory storage interfaced bymemory interface unit 110. In general, memory includes any suitablememory hierarchy including caches, addressable read/write memorystorage, secondary storage, etc. Dribble manager 151 also provides thenecessary control signals for a single read port and a single write portof stack cache 155 which are used exclusively for background dribblingpurposes.

In one embodiment, stack cache 155 is managed as a circular buffer whichensures that the stack grows and shrinks in a predictable manner toavoid overflows or overwrites. The saving and restoring of values to andfrom data cache 165 is controlled by dribbler manager 151 using high-and low-water marks, in one embodiment.

Stack management unit 150 provides execution unit 140 with two 32-bitoperands in a given cycle. Stack management unit 150 can store a single32-bit result in a given cycle.

Dribble manager 151 handles spills and fills of stack cache 155 byspeculatively dribbling the data in and out of stack cache 155 from andto data cache 165. Dribble manager 151 generates a pipeline stall signalto stall the pipeline when a stack overflow or underflow condition isdetected. Dribble manager 151 also keeps track of requests sent to datacache unit 160. A single request to data cache unit 160 is a 32-bitconsecutive load or store request.

The hardware organization of stack cache 155 is such that, except forlong operands (long integers and double precision floating-pointnumbers), implicit operand fetches for opcodes do not add latency to theexecution of the opcodes. The number of entries in operand stack 423(FIG. 4A) and local variable storage 421 that are maintained in stackcache 155 represents a hardware/performance tradeoff. At least a fewoperand stack 423 and local variable storage 422 entries are required toget good performance. In the exemplary embodiment of FIG. 1, at leastthe top three entries of operand stack 423 and the first four localvariable storage 421 entries are preferably represented in stack cache155.

One key function provided by stack cache 155 (Fig. 1) is to emulate aregister file where access to the top two registers is always possiblewithout extra cycles. A small hardware stack is sufficient if the properintelligence is provided to load/store values from/to memory in thebackground, therefore preparing stack cache 155 for incoming virtualmachine instructions.

As indicated above, all items on stack 400 (regardless of size) areplaced into a 32-bit word. This tends to waste space if many small dataitems are used, but it also keeps things relatively simple and free oflots of tagging or muxing. An entry in stack 400 thus represents a valueand not a number of bytes. Long integer and double precisionfloating-point numbers require two entries. To keep the number of readand write ports low, two cycles to read two long integers or two doubleprecision floating point numbers are required.

The mechanism for filling and spilling the operand stack from stackcache 155 out to memory by dribble manager 151 can assume one of severalalternative forms. One register at a time can be filled or spilled, or ablock of several registers filled or spilled at once. A simplescoreboarded method is appropriate for stack management. In its simplestform, a single bit indicates if the register in stack cache 155 iscurrently valid. In addition, some embodiments of stack cache 155 use asingle bit to indicate whether the data content of the register is savedto stack 400, i.e., whether the register is dirty. In one embodiment, ahigh-water mark/low-water mark heuristic determines when entries aresaved to and restored from stack 400, respectively (FIG. 4A).Alternatively, when the top-of-the-stack becomes close to bottom 401 ofstack cache 155 by a fixed, or alternatively, a programmable number ofentries, the hardware starts loading registers from stack 400 into stackcache 155. Detailed embodiments of stack management unit 150 and dribblemanager unit 151 are described below and in U.S. patent application Ser.No. 08/789,617, entitled "METHOD FRAME STORAGE USING MULTIPLE MEMORYCIRCUITS" naming James Michael O'Connor and Marc Tremblay as inventors,assigned to the assignee of this application, and filed on even dateherewith, which is incorporated herein by reference in its entirety.

In one embodiment, stack management unit 150 also includes an optionallocal variable look-aside cache 153. Cache 153 is most important inapplications where both the local variables and operand stack 423 (FIG.4A) for a method are not located on stack cache 155. In such instanceswhen cache 153 is not included in hardware processor 100, there is amiss on stack cache 155 when a local variable is accessed, and executionunit 140 accesses data cache unit 160, which in turn slows downexecution. In contrast, with cache 153, the local variable is retrievedfrom cache 153 and there is no delay in execution.

One embodiment of local variable look-aside cache 153 is illustrated inFIG. 4D for method 0 to 2 on stack 400. Local variables zero to M, whereM is an integer, for method 0 are stored in plane 421A₋₋ 0 of cache 153and plane 421A₋₋ 0 is accessed when method number 402 is zero. Localvariables zero to N, where N is an integer, for method 1 are stored inplane 421A₋₋ 1 of cache 153 and plane 421A₋₋ 1 is accessed when methodnumber 402 is one. Local variables zero to P, where P is an integer, formethod 1 are stored in plane 421A₋₋ 2 of cache 153 and plane 421A₋₋ 2 isaccessed when method number 402 is two. Notice that the various planesof cache 153 may be different sizes, but typically each plane of thecache has a fixed size that is empirically determined.

When a new method is invoked, e.g, method 2, a new plane 421A₋₋ 2 incache 153 is loaded with the local variables for that method, and methodnumber register 402, which in one embodiment is a counter, is changed,e.g., incremented, to point to the plane of cache 153 containing thelocal variables for the new method. Notice that the local variables areordered within a plane of cache 153 so that cache 153 is effectively adirect-mapped cache. Thus, when a local variable is needed for thecurrent method, the variable is accessed directly from the most recentplane in cache 153, i.e., the plane identified by method number 402.When the current method returns, e.g., method 2, method number register402 is changed, e.g., decremented, to point at previous plane 421A-1 ofcache 153. Cache 153 can be made as wide and as deep as necessary.

Data Cache Unit

Data cache unit 160 (DCU) manages all requests for data in data cache165. Data cache requests can come from dribbling manager 151 orexecution unit 140. Data cache controller 161 arbitrates between theserequests giving priority to the execution unit requests. In response toa request, data cache controller 161 generates address, data and controlsignals for the data and tags RAMs in data cache 165. For a data cachehit, data cache controller 161 reorders the data RAM output to providethe right data.

Data cache controller 161 also generates requests to I/O bus and memoryinterface unit 110 in case of data cache misses, and in case ofnon-cacheable loads and stores. Data cache controller 161 provides thedata path and control logic for processing non-cacheable requests, andthe data path and data path control functions for handling cache misses.

For data cache hits, data cache unit 160 returns data to execution unit140 in one cycle for loads. Data cache unit 160 also takes one cycle forwrite hits. In case of a cache miss, data cache unit 160 stalls thepipeline until the requested data is available from the external memory.For both non-cacheable loads and stores, data cache 165 is bypassed andrequests are sent to I/O bus and memory interface unit 110. Non-alignedloads and stores to data cache 165 trap in software.

Data cache 165 is a two-way set associative, write back, write allocate,16-byte line cache. The cache size is configurable to 0, 1, 2, 4, 8, 16Kbyte sizes. The default size is 8 Kbytes. Each line has a cache tagstore entry associated with the line. On a cache miss, 16 bytes of dataare written into cache 165 from external memory.

Each data cache tag contains a 20-bit address tag field, one valid bit,and one dirty bit. Each cache tag is also associated with a leastrecently used bit that is used for replacement policy. To supportmultiple cache sizes, the width of the tag fields also can be varied. Ifa cache enable bit in processor service register is not set, loads andstores are treated like non-cacheable instructions by data cachecontroller 161.

A single sixteen-byte write back buffer is provided for writing backdirty cache lines which need to be replaced. Data cache unit 160 canprovide a maximum of four bytes on a read and a maximum of four bytes ofdata can be written into cache 165 in a single cycle. Diagnostic readsand writes can be done on the caches.

Memory Allocation Accelerator

In one embodiment, data cache unit 160 includes a memory allocationaccelerator 166. Typically, when a new object is created, fields for theobject are fetched from external memory, stored in data cache 165 andthen the field is cleared to zero. This is a time consuming process thatis eliminated by memory allocation accelerator 166. When a new object iscreated, no fields are retrieved from external memory. Rather, memoryallocation accelerator 166 simply stores a line of zeros in data cache165 and marks that line of data cache 165 as dirty. Memory allocationaccelerator 166 is particularly advantageous with a write-back cache.Since memory allocation accelerator 166 eliminates the external memoryaccess each time a new object is created, the performance of hardwareprocessor 100 is enhanced.

Floating Point Unit

Floating point unit (FPU) 143 includes a microcode sequencer,input/output section with input/output registers, a floating pointadder, i.e., an ALU, and a floating point multiply/divide unit. Themicrocode sequencer controls the microcode flow and microcode branches.The input/output section provides the control for input/output datatransactions, and provides the input data loading and output dataunloading registers. These registers also provide intermediate resultstorage.

The floating point adder-ALU includes the combinatorial logic used toperform the floating point adds, floating point subtracts, andconversion operations. The floating point multiply/divide unit containsthe hardware for performing multiply/divide and remainder.

Floating point unit 143 is organized as a microcoded engine with a32-bit data path. This data path is often reused many times during thecomputation of the result. Double precision operations requireapproximately two to four times the number of cycles as single precisionoperations. The floating point ready signal is asserted one-cycle priorto the completion of a given floating point operation. This allowsinteger unit 142 to read the floating point unit output registerswithout any wasted interface cycles. Thus, output data is available forreading one cycle after the floating point ready signal is asserted.

Execution Unit Accelerators

Since the JAVA Virtual Machine Specification of Appendix I is hardwareindependent, the virtual machine instructions are not optimized for aparticular general type of processor, e.g., a complex instruction setcomputer (CISC) processor, or a reduced instruction set computer (RISC)processor. In fact, some virtual machine instructions have a CISC natureand others a RISC nature. This dual nature complicates the operation andoptimization of hardware processor 100.

For example, the JAVA virtual machine specification defines opcode 171for an instruction lookupswitch, which is a traditional switchstatement. The datastream to instruction cache unit 120 includes anopcode 171, identifying the N-way switch statement, that is followedzero to three bytes of padding. The number of bytes of padding isselected so that first operand byte begins at an address that is amultiple of four. Herein, datastream is used generically to indicateinformation that is provided to a particular element, block, component,or unit.

Following the padding bytes in the datastream are a series of pairs ofsigned four-byte quantities. The first pair is special. A first operandin the first pair is the default offset for the switch statement that isused when the argument, referred to as an integer key, or alternatively,a current match value, of the switch statement is not equal to any ofthe values of the matches in the switch statement. The second operand inthe first pair defines the number of pairs that follow in thedatastream.

Each subsequent operand pair in the datastream has a first operand thatis a match value, and a second operand that is an offset. If the integerkey is equal to one of the match values, the offset in the pair is addedto the address of the switch statement to define the address to whichexecution branches. Conversely, if the integer key is unequal to any ofthe match values, the default offset in the first pair is added to theaddress of the switch statement to define the address to which executionbranches. Direct execution of this virtual machine instruction requiresmany cycles.

To enhance the performance of hardware processor 100, a look-up switchaccelerator 145 is included in hardware processor 100. Look-up switchaccelerator 145 includes an associative memory which stores informationassociated with one or more lookup switch statements. For each lookupswitch statement, i.e., each instruction lookupswitch, this informationincludes a lookup switch identifier value, i.e., the program countervalue associated with the lookup switch statement, a plurality of matchvalues and a corresponding plurality of jump offset values.

Lookup switch accelerator 145 determines whether a current instructionreceived by hardware processor 100 corresponds to a lookup switchstatement stored in the associative memory. Lookup switch accelerator145 further determines whether a current match value associated with thecurrent instruction corresponds with one of the match values stored inthe associative memory. Lookup switch accelerator 145 accesses a jumpoffset value from the associative memory when the current instructioncorresponds to a lookup switch statement stored in the memory and thecurrent match value corresponds with one of the match values stored inthe memory wherein the accessed jump offset value corresponds with thecurrent match value.

Lookup switch accelerator 145 further includes circuitry for retrievingmatch and jump offset values associated with a current lookup switchstatement when the associative memory does not already contain the matchand jump offset values associated with the current lookup switchstatement. Lookup switch accelerator 145 is described in more detail inU.S. patent application Ser. No. 08/788,811, entitled "LOOK-UP SWITCHACCELERATOR AND METHOD OF OPERATING SAME" naming Marc Tremblay and JamesMichael O'Connor as inventors, assigned to the assignee of thisapplication, and filed on even date herewith, which is incorporatedherein by reference in its entirety.

In the process of initiating execution of a method of an object,execution unit 140 accesses a method vector to retrieve one of themethod pointers in the method vector, i.e., one level of indirection.Execution unit 140 then uses the accessed method pointer to access acorresponding method, i.e., a second level of indirection.

To reduce the levels of indirection within execution unit 140, eachobject is provided with a dedicated copy of each of the methods to beaccessed by the object. Execution unit 140 then accesses the methodsusing a single level of indirection. That is, each method is directlyaccessed by a pointer which is derived from the object. This eliminatesa level of indirection which was previously introduced by the methodpointers. By reducing the levels of indirection, the operation ofexecution unit 140 can be accelerated. The acceleration of executionunit 140 by reducing the levels of indirection experienced by executionunit 140 is described in more detail in U.S. patent application Ser. No.08/787,846, entitled "REPLICATING CODE TO ELIMINATE A LEVEL OFINDIRECTION DURING EXECUTION OF AN OBJECT ORIENTED COMPUTER PROGRAM"naming Marc Tremblay and James Michael O'Connor as inventors, assignedto the assignee of this application, and filed on even date herewith,which is incorporated herein by reference in its entirety.

Getfield-putfield Accelerator

Other specific functional units and various translation lookaside buffer(TLB) types of structures may optionally be included in hardwareprocessor 100 to accelerate accesses to the constant pool. For example,the JAVA virtual machine specification defines an instruction putfield,opcode 181, that upon execution sets a field in an object and aninstruction getfield, opcode 180, that upon execution fetches a fieldfrom an object. In both of these instructions, the opcode is followed byan index byte one and an index byte two. Operand stack 423 contains areference to an object followed by a value for instruction putfield, butonly a reference to an object for instruction getfield.

Index bytes one and two are used to generate an index into the constantpool of the current class. The item in the constant pool at that indexis a field reference to a class name and a field name. The item isresolved to a field block pointer which has both the field width, inbytes, and the field offset, in bytes.

An optional getfield-putfield accelerator 146 in execution unit 140stores the field block pointer for instruction getfield or instructionputfield in a cache, for use after the first invocation of theinstruction, along with the index used to identify the item in theconstant pool that was resolved into the field block pointer as a tag.Subsequently, execution unit 140 uses index bytes one and two togenerate the index and supplies the index to getfield-putfieldaccelerator 146. If the index matches one of the indexes stored as atag, i.e., there is a hit, the field block pointer associated with thattag is retrieved and used by execution unit 140. Conversely, if a matchis not found, execution unit 140 performs the operations describedabove. Getfield-putfield accelerator 146 is implemented without usingselfmodifying code that was used in one embodiment of the quickinstruction translation described above.

In one embodiment, getfield-putfield accelerator 146 includes anassociative memory that has a first section that holds the indices thatfunction as tags, and a second section that holds the field blockpointers. When an index is applied through an input section to the firstsection of the associative memory, and there is a match with one of thestored indices, the field block pointer associated with the stored indexthat matched in input index is output from the second section of theassociative memory.

Bounds Check Unit

Bounds check unit 147 (FIG. 1) in execution unit 140 is an optionalhardware circuit that checks each access to an element of an array todetermine whether the access is to a location within the array. When theaccess is to a location outside the array, bounds check unit 147 issuesan active array bound exception signal to execution unit 140. Inresponse to the active array bound exception signal, execution unit 140initiates execution of an exception handler stored in microcode ROM 141that in handles the out of bounds array access.

In one embodiment, bounds check unit 147 includes an associative memoryelement in which is stored a array identifier for an array, e.g., aprogram counter value, and a maximum value and a minimum value for thearray. When an array is accessed, i.e., the array identifier for thatarray is applied to the associative memory element, and assuming thearray is represented in the associative memory element, the storedminimum value is a first input signal to a first comparator element,sometimes called a comparison element, and the stored maximum value is afirst input signal to a second comparator element, sometimes also calleda comparison element. A second input signal to the first and secondcomparator elements is the value associated with the access of thearray's element.

If the value associated with the access of the array's element is lessthan or equal to the stored maximum value and greater than or equal tothe stored minimum value, neither comparator element generates an outputsignal. However, if either of these conditions is false, the appropriatecomparator element generates the active array bound exception signal. Amore detailed description of one embodiment of bounds check unit 147 isprovided in U.S. patent application Ser. No. 08/786,352, entitled"PROCESSOR WITH ACCELERATED ARRAY ACCESS BOUNDS CHECKING" naming MarcTremblay, James Michael O'Connor, and William N. Joy as inventors,assigned to the assignee of this application, and filed on even dateherewith which is incorporated herein by reference in its entirety.

The JAVA Virtual Machine Specification defines that certain instructionscan cause certain exceptions. The checks for these exception conditionsare implemented, and a hardware/software mechanism for dealing with themis provided in hardware processor 100 by information in microcode ROM141 and program counter and trap control logic 170. The alternativesinclude having a trap vector style or a single trap target and pushingthe trap type on the stack so that the dedicated trap handler routinedetermines the appropriate action.

No external cache is required for the architecture of hardware processor100. No translation lookaside buffers need be supported.

FIG. 5 illustrates several possible add-ons to hardware processor 100 tocreate a unique system. Circuits supporting any of the eight functionsshown, i.e., NTSC encoder 501, MPEG 502, Ethernet controller 503, VIS504, ISDN 505, I/O controller 506, ATM assembly/reassembly 507, andradio link 508 can be integrated into the same chip as hardwareprocessor 100 of this invention.

FIG. 6 is a block diagram of one embodiment of a stack management unit150. Stack management unit 150 serves as a high speed buffer betweenstack 400 and hardware processor 100. Hardware processor 100 accessesstack management unit 150 as if stack management unit 150 were stack400. Stack management unit 150 automatically transfers data betweenstack management unit 150 and stack 400 as necessary to provide improvethe throughput of data between stack 400 and hardware processor 100. Inthe embodiment of FIG. 1, if hardware processor 100 requires a data wordwhich is not cached in stack management unit 150, data cache unit 160retrieves the requested data word and places the requested data word atthe top of stack cache 155.

Stack management unit 150 contains a stack cache memory circuit 610.Stack cache memory circuit 610 is typically fast memory devices such asa register file or SRAM; however, slower memory devices such as DRAM canalso be used. In the embodiment of FIG. 6, access to stack cache memorycircuit 610 is controlled by stack control unit 152. A write port 630allows hardware processor 100 to write data on data lines 635 to stackcache memory circuit 610. Read port 640 and read port 650 allow hardwareprocessor 100 to read data from stack cache memory circuit 610 on datalines 645 and 655, respectively. Two read ports are provided to increasethroughput since many operations of stack-based computing systemsrequire two operands from stack 400. Other embodiments of stack cache155 may provide more or less read and write ports.

As explained above, dribble manager unit 151 controls the transfer ofdata between stack 400 (FIG. 4(a)) and stack cache memory circuit 610.In the embodiment shown in FIG. 1, the transfer of data between stack400 and stack cache memory circuit 610 goes through data cache unit 160.Dribble manager unit 151 includes a fill control unit 694 and a spillcontrol unit 698. In some embodiments of dribble manager unit 151, fillcontrol unit 694 and spill control unit 698 function independently. Fillcontrol unit 694 determines if a fill condition exists. If the fillcondition exists, fill control unit 694 transfers data words from stack400 to stack cache memory circuit 610 on data lines 675 through a writeport 670. Spill control unit 698 determines if a spill condition exists.If the spill condition exists, spill control unit 698 transfers datawords from stack cache memory circuit 610 to stack 400 through read port680 on data lines 685. Write port 670 and read port 680 allows transfersbetween stack 400 and stack cache memory circuit 610 to occursimultaneously with reads and writes controlled by stack control unit152. If contention for read and write ports of stack cache memorycircuit 610 is not important, dribble manager unit 151 can share readand write ports with stack control unit 152.

Although stack management unit 150 is described in the context ofbuffering stack 400 for hardware processor 100, stack management unit150 can perform caching for any stack-based computing system. Thedetails of hardware processor 100 are provided only as an example of onepossible stack-based computing system for use with the presentinvention. Thus, one skilled in the art can use the principles describedherein to design a stack management unit in accordance to the presentinvention for any stack-based computing system.

FIG. 7 shows a conceptual model of the memory architecture of stackcache memory circuit 610 for one embodiment of stack cache 155.Specifically, in the embodiment of FIG. 7, stack cache memory circuit610 is a register file organized in a circular buffer memoryarchitecture capable of holding 64 data words. Other embodiments maycontain a different number of data words. The circular memoryarchitecture causes data words in excess of the capacity of stack cachememory circuit 610 to be written to previously used registers. If stackcache memory unit 610 uses a different memory device, such as an SRAM,different registers would correspond to different memory locations. Onetechnique to address registers in a circular buffer is to use pointerscontaining modulo stack cache size (modulo-SCS) addresses to the variousregisters of stack cache memory circuit 610. As used herein, modulo-Noperations have the results of the standard operation mapped to a numberbetween 0 and SCS-1 using a standard MOD N function. Some common modulooperations are defined as follows

Modulo-N addition of X and Y=(X+Y) MOD N,

Modulo-N subtraction of x and Y=(X-Y) MOD N,

Modulo-N increment of X by Y=(X+Y) MOD N,

Modulo-N decrement of X by Y=(X-Y) MOD N.

One embodiment of the pointer addresses of the registers of stack cachememory circuit 610 are shown in FIG. 7 as numbered 0-63 along the outeredge of stack cache memory circuit 610. Thus for the embodiment of FIG.7, if 70 data words (numbered 1 to 70) are written to stack cache memorycircuit 610 when stack cache memory circuit 610 is empty, data words 1to 64 are written to registers 0 to 63, respectively and data words 65to 70 are written subsequently to registers 0 to 5. Prior to writingdata words 65 to 70, dribble manager unit 151, as described below,transfers data words 1 to 6 which were in registers 0 to 5 to stack 400.Similarly, as data words 70 to 65 are read out of stack cache memorycircuit 610, data words 1 to 6 can be retrieved from stack 400 andplaced in memory locations 0 to 5.

Since most reads and writes on a stack are from the top of the stack, apointer OPTOP contains the location of the top of stack 400, i.e. thetop memory location. In some embodiments of stack management unit 150,pointer OPTOP is a programmable register in execution unit 140. Howeverother embodiments of stack management unit 150 maintain pointer OPTOP instack control unit 152. Since pointer OPTOP is often increased by one,decreased by one, or changed by a specific amount, pointer OPTOP, in oneembodiment is a programmable up/down counter.

Since stack management unit 150 contains the top portion of stack 400,pointer OPTOP indicates the register of stack cache memory circuit 610containing the most recently written data word in stack cache memorycircuit 610, i.e. pointer OPTOP points to the register containing themost recently written data word also called the top register. Someembodiments of stack management unit 150 also contains a pointer OPTOP1(not shown) which points to the register preceding the register pointedto by pointer OPTOP. Pointer OPTOP1 can improve the performance of stackmanagement unit 150 since many operations in hardware processor 100require two data words from stack management unit 150.

Pointer OPTOP and pointer OPTOP1 are incremented whenever a new dataword is written to stack cache 155. Pointer OPTOP and pointer OPTOP1 aredecremented whenever a stacked data word, i.e. a data word already instack 400, is popped off of stack cache 155. Since some embodiments ofhardware processor 100 may add or remove multiple data wordssimultaneously, pointer OPTOP and OPTOP1 are implemented, in oneembodiment as programmable registers so that new values can be writteninto the registers rather than requiring multiple increment or decrementcycles.

If stack cache 155 is organized using sequential addressing, pointerOPTOP1 may also be implemented using a modulo SCS subtractor whichmodulo-SCS subtracts one from pointer OPTOP. Some embodiments of stackcache 155 may also include pointers OPTOP2 or pointer OPTOP3.

Since data words are stored in stack cache memory circuit 610circularly, the bottom of stack cache memory circuit 610 can fluctuate.Therefore, most embodiments of stack cache memory circuit 610 include apointer CACHE₋₋ BOTTOM to indicate the bottom memory location of stackcache memory circuit 610. Pointer CACHE₋₋ BOTTOM is typically maintainedby dribble manager unit 151. The process to increment or decrementpointer CACHE₋₋ BOTTOM varies with the specific embodiment of stackmanagement unit 150. Pointer CACHE₋₋ BOTTOM is typically implemented asa programmable up/down counter.

Some embodiments of stack management unit 150 also includes otherpointers, such as pointer VARS, which points to a memory location of adata word that is often accessed. For example, if hardware processor 100is implementing the JAVA Virtual Machine, entire method frames may beplaced in stack management unit 150. The method frames often containlocal variables that are accessed frequently. Therefore, having pointerVARS pointed to the first local variable of the active method decreasesthe access time necessary to read the local variable. Other pointerssuch as a pointer VARS1 (not shown) and a pointer VARS2 (not shown) maypoint to other often used memory locations such as the next two localvariables of the active method in a JAVA Virtual Machine. In someembodiments of stack management unit 150, these pointers are maintainedin stack control unit 152. In embodiments adapted for use with hardwareprocessor 100, pointer VARS is stored in a programmable register inexecution unit 140. If stack cache 155 is organized using sequentialaddressing, pointer VARS1 may also be implemented using a modulo-SCSadder which modulo-SCS adds one to pointer VARS.

To determine which data words to transfer between stack cache memorycircuit 610 and stack 400, stack management unit 150, typically tags,i.e. tracks, the valid data words and the data words which are stored inboth stack cache memory circuit 610 and stack 400. FIG. 8 illustratesone tagging scheme used in some embodiments of stack management unit150. Specifically, FIG. 8 shows a register 810 from stack cache memorycircuit 610. The actual data word is stored in data section 812. A validbit 814 and a saved bit 816 are used to track the status of register810. If valid bit 814 is at a valid logic state, typically logic high,data section 812 contains a valid data word. If valid bit 814 is at aninvalid logic state, typically logic low, data section 812 does notcontain a valid data word. If saved bit 816 is at a saved logic state,typically logic high, the data word contained in data section 812 isalso stored in stack 400. However, if saved bit 816 is at an unsavedlogic state, typically logic low, the data word contained in datasection 812 is not stored in stack 400. Typically, when stack managementunit 150 is powered up or reset, valid bit 814 of each register is setto the invalid logic state and saved bit 816 of each register is set tothe unsaved logic state.

For the embodiment illustrated in FIG. 6 using the tagging method ofFIG. 8, when stack control unit 152 writes a data word to a register instack cache memory circuit 610 through write port 630 the valid bit ofthat register is set to the valid logic state and the saved bit of thatregister is set to the unsaved logic state. When dribble manager unit151 transfer a data word to a register of stack cache memory circuit 610through write port 670, the valid bit of that register is set to thevalid logic state and the saved bit of that register is set to the savedlogic state since the data word is currently saved in stack 400.

When hardware processor 100 reads a stacked data word using a stackpopping operation from a register of stack cache memory circuit 610through either read port 640 or read port 650 the valid bit of thatregister is set to the invalid logic state and the saved bit of thatlocation is set to the unsaved logic state. Typically, stack poppingoperations use the register indicated by pointer OPTOP or pointerOPTOP1.

When hardware processor 100 reads a data word with a non-stack poppingoperation from a register of stack cache memory circuit 610 througheither read port 640 or read port 650 the valid bit and saved bit of theregister are not changed. For example, if hardware processor 100 isimplementing the JAVA Virtual Machine, a local variable stored in stackcache memory circuit 610 in the register indicated by pointer VARS maybe used repeatedly and should not be removed from stack cache 155. Whendribble manager unit 151 copies a data word from a register of stackcache memory circuit 610 to stack 400 through read port 680, the validbit of that register remains in the valid logic state since the saveddata word is still contained in that register and the saved bit of thatregister is set to the saved logic state.

Since stack cache 155 is generally much smaller than the memory addressspace of hardware processor 100, the pointers used to access stack cachememory circuit 610 are generally much smaller than general memoryaddresses. The specific technique used to map stack cache 155 into thememory space of hardware processor 100 can vary. In one embodiment ofhardware processor 100 the pointers used to access stack cache memorycircuit 610 are only the lower bits of general memory pointers, i.e. theleast significant bits. For example, if stack cache memory circuit 610comprises 64 registers, pointers OPTOP, VARS, and CACHE₋₋ BOTTOM needonly be six bits long. If hardware processor 100 has a 12 bit addressspace, pointers OPTOP, VARS, and CACHE₋₋ BOTTOM could be the lower sixbits of a general memory pointer. Thus stack cache memory circuit 610 ismapped to a specific segment of the address space having a unique uppersix bit combination.

Some embodiments of stack cache management unit 150 may be used withpurely stacked based computing system so that there is not a memoryaddress space for the system. In this situation, the pointers foraccessing stack cache 155 are only internal to stack cache managementunit 150.

As explained above, hardware processor 100 primarily accesses data nearthe top of the stack. Therefore, stack management unit 150 can improvedata accesses of hardware processor 100 while only caching the topportion of stack 400. When hardware processor 100 pushes more data wordsto stack management unit 150 than stack cache memory circuit 610 is ableto store, the data words near the bottom of stack cache memory circuit610 are transferred to stack 400. When hardware processor 100 pops datawords out of stack cache 155, data words from stack 400 are copied underthe bottom of stack cache memory circuit 610, and pointer CACHE₋₋ BOTTOMis decremented to point to the new bottom of stack cache memory circuit610.

Determination of when to transfer data words between stack 400 and stackcache memory circuit 610 as well as how many data words to transfer canvary. In general, dribble manager unit 151 should transfer data fromstack cache memory circuit 610 to stack 400, i.e. a spill operation, ashardware processor fills stack cache memory circuit 610. Conversely,dribble manager unit 151 should copy data from stack 400 to stack cachememory circuit 610, i.e. a fill operation, as hardware processor emptiesstack cache memory circuit 610.

FIG. 9 shows one embodiment of dribble manager unit 151 in whichdecisions on transferring data from stack cache memory circuit 610 tostack 400, i.e. spilling data, are based on the number of free registersin stack cache memory circuit 610. Free registers includes registerswithout valid data as well as registers containing data already storedin stack 400, i.e. registers with saved bit 816 set to the saved logicstate. Decisions on transferring data from stack 400 to stack cachememory circuit 610, i.e. filling data, are based on the number of usedregisters. A used registers contains a valid but unsaved data word instack cache memory circuit 610.

Specifically in the embodiment of FIG. 9, dribble manager unit 151further includes a stack cache status circuit 910 and a cache bottomregister 920, which can be a programmable up/down counter. Stack cachestatus circuit 910, receives pointer CACHE₋₋ BOTTOM from cache bottomregister 920 and pointer OPTOP to determine the number of free registersFREE and the number of used registers USED.

For a circular buffer using sequential modulo-SCS addressing, as in FIG.7, the number of free registers FREE is defined as

FREE=SCS-(OPTOP-CACHE₋₋ BOTTOM+1) MOD SCS, where SCS is the size ofstack cache 155. Thus, for the specific pointer values shown in FIG. 7,the number of free registers FREE is 34, as calculated by:

FREE=64-((27-62+1) MOD 64)=34.

Similarly, for a circular buffer using sequential modulo addressing, thenumber of used registers USED defined as

USED=(OPTOP-CACHE₋₋ BOTTOM+1) MOD SCS. Thus, for the specific pointervalues shown in FIG. 7, the number of used registers USED is 30, ascalculated by:

USED=(27-62+1) MOD 64.

Thus, stack cache status circuit 910 can be implemented with a moduloSCS adder/subtractor. The number of used registers USED and the numberof free registers FREE can also be generated using a programmableup/down counters. For example, a used register can be incrementedwhenever a data word is added to stack cache 155 and decrementedwhenever a data word is removed from stack cache 155. Specifically, ifpointer OPTOP is modulo-SCS incremented by some amount, the usedregister is incremented by the same amount. If pointer OPTOP ismodulo-SCS decremented by some amount, the used register is decrementedby the same amount. However, if pointer CACHE₋₋ BOTTOM is modulo-SCSincremented by some amount, the used register is decremented by the sameamount. If pointer CACHE₋₋ BOTTOM is modulo-SCS decremented by someamount, the used register is incremented the same amount. The number offree registers FREE can be generated by subtracting the number of usedregisters USED from the total number of registers.

Spill control unit 694 (FIGS. 6 and 9) includes a cache high thresholdregister 930 and a comparator 940. Comparator 940 compares the value incache high threshold register 930 to the number of free registers FREE.If the number of free registers FREE is less than the value in cachehigh threshold register 930, comparator 940 drives a spill signal SPILLto a spill logic level, typically logic high, to indicate that the spillcondition exists and one or more data words should be transferred fromstack cache memory circuit 610 to stack 400, i.e. a spill operationshould be performed. The spill operation is described in more detailbelow. Typically, cache high threshold register 930 is programmable byhardware processor 100.

Fill control unit 698 (FIGS. 6 and 9) includes a cache low thresholdregister 950 and a comparator 960. Comparator 960 compares the value incache low threshold register 950 to the number of used registers USED.If the number of used registers is less than the value in cache lowthreshold register 950, comparator 960 drives a fill signal FILL to afill logic level, typically logic high, to indicate that the fillcondition exists and one or more data words should be transferred fromstack 400 to stack cache memory circuit 610, i.e. a fill operationshould be performed. The fill operation is described in more detailbelow. Typically, cache low threshold register 950 is programmable byhardware processor 100.

If the value in cache high threshold 930 and cache low threshold 940 isalways the same, a single cache threshold register can be used. Fillcontrol unit 698 can be modified to use the number of free registersFREE to drive signal FILL to the fill logic level if then number of freeregisters is greater than the value in cache low threshold 950, with aproper modification of the value in cache low threshold 950.Alternatively, spill control unit 694 can be modified to use the numberof used registers.

FIG. 10A shows another embodiment of dribble manager unit 151, whichuses a high-water mark/low-water mark heuristic to determine when aspill condition or a fill condition exists. Spill control unit 694includes a high water mark register 1010 implemented as a programmableup/down counter. A comparator 1020 in spill control unit 694 comparesthe value in high water mark register 1010, i.e. the high water mark,with pointer OPTOP. If pointer OPTOP is greater than the high watermark, comparator 1020 drives spill signal SPILL to the spill logic levelto indicate a spill operation should be performed. Since, the high watermark is relative to pointer CACHE₋₋ BOTTOM, the high water mark ismodulo-SCS incremented and modulo-SCS decremented whenever pointerCACHE₋₋ BOTTOM is modulo-SCS incremented or modulo-SCS decremented,respectively.

Fill control unit 698 includes a low water mark register 1010implemented as a programmable up/down counter. A comparator 1030 in fillcontrol unit 698 compares the value in low water mark register 1030,i.e. the low water mark, with pointer OPTOP. If pointer OPTOP is lessthan the low water mark, comparator 1040 drives fill signal FILL to thefill logic level to indicate a fill operation should be performed. Sincethe low water mark is relative to pointer CACHE₋₋ BOTTOM, the low watermark register is modulo-SCS incremented and modulo-SCS decrementedwhenever pointer CACHE₋₋ BOTTOM is modulo-SCS incremented or modulo-SCSdecremented, respectively.

FIG. 10B shows an alternative circuit to generate the high water markand low water mark. Cache high threshold register 930, typicallyimplemented as a programmable register, contains the number of freeregisters which should be maintained in stack cache memory circuit 610.The high water mark is then calculated by modulo-SCS subtractor 1050 bymodulo-SCS subtracting the value in cache high threshold register 930from pointer CACHE₋₋ BOTTOM stored in cache bottom register 920.

The low water mark is calculated by doing a modulo-SCS addition.Specifically, cache low threshold register 950 is programmed to containthe minimum number of used data registers desired to be maintained instack cache memory circuit 610. The low water mark is then calculated bymodulo-SCS adder 1060 by modulo-SCS adding the value in cache lowthreshold register 950 with pointer CACHE₋₋ BOTTOM stored in cachebottom register 920.

As described above, a spill operation is the transfer of one or moredata words from stack cache memory circuit 610 to stack 400. In theembodiment of FIG. 1, the transfers occurs though data cache unit 160.The specific interface between stack management unit 150 and data cacheunit 160 can vary. Typically, stack management unit 150, and morespecifically dribble manager unit 151, sends the data word located atthe bottom of stack cache 155, as indicated by pointer CACHE₋₋ BOTTOMfrom read port 680 to data cache unit 160. The value of pointer CACHE₋₋BOTTOM is also provided to data cache unit 160 so that data cache unit160 can address the data word appropriately. The saved bit of theregister indicated by pointer CACHE₋₋ BOTTOM is set to the saved logiclevel. In addition, pointer CACHE₋₋ BOTTOM is modulo-SCS incremented byone. Other registers as described above may also be modulo-SCSincremented by one. For example, high water mark register 1010 (FIG. 10A)and low water mark 1030 would be modulo-SCS incremented by one. Someembodiments of dribble manager unit 151 transfer multiple words for eachspill operation. For these embodiments, pointer CACHE₋₋ BOTTOM ismodulo-SCS incremented by the number words transferred to stack 400.

In embodiments using a saved bit and valid bit, as shown in FIG. 8, someoptimization is possible. Specifically, if the saved bit of the dataregister pointed to by pointer CACHE₋₋ BOTTOM is at the saved logiclevel, the data word in that data register is already stored in stack400. Therefore, the data word in that data register does not need to becopied to stack 400. However, pointer CACHE₋₋ BOTTOM is still modulo-SCSincremented by one.

A fill operation transfers data words from stack 400 to stack cachememory circuit 610. In the embodiment of FIG. 1, the transfers occursthough data cache unit 160. The specific interface between stackmanagement unit 150 and data cache unit 160 can vary. Typically, stackmanagement unit 150, and more specifically dribble manager unit 151,determines whether the data register preceding the data register pointedby CACHE₋₋ BOTTOM is free, i.e. either the saved bit is in the savedlogic state or the valid bit is in the invalid logic state. If the dataregister preceding the data register pointed to by pointer CACHE₋₋BOTTOM is free, dribble manager unit 151 requests a data word from stack400 by sending a request with the value of pointer CACHE₋₋ BOTTOMmodulo-SCS minus one. When the data word is received from data cacheunit 160, pointer CACHE₋₋ BOTTOM is modulo-SCS decremented by one andthe received data word is written to the data register pointed to bypointer CACHE₋₋ BOTTOM through write port 670. Other registers asdescribed above may also be modulo-SCS decremented. The saved bit andvalid bit of the register pointed to by pointer CACHE₋₋ BOTTOM are setto the saved logic state and valid logic state, respectively. Someembodiments of dribble manager unit 151 transfer multiple words for eachspill operation. For these embodiments, pointer CACHE₋₋ BOTTOM ismodulo-SCS decremented by the number words transferred to stack 400.

In embodiments using a saved bit and valid bit, as shown in FIG. 8, someoptimization is possible. Specifically, if the saved bit and valid bitof the data register preceding the data register pointed to by pointerCACHE₋₋ BOTTOM is at the saved logic level and the valid logic level,respectively, then the data word in that data register was neveroverwritten. Therefore, the data word in that data register does notneed to be copied from stack 400. However, pointer CACHE₋₋ BOTTOM isstill modulo-SCS decremented by one.

As stated above, in one embodiment of stack cache 155, hardwareprocessor 100 accesses stack cache memory circuit 610 (FIG. 6) throughwrite port 630, read port 640 and read port 650. Stack control unit 152generates pointers for write port 630, read port 640, and read port 650based on the requests of hardware processor 100. FIG. 11 shows a circuitto generate pointers for a typical operation which reads two data wordsfrom stack cache 155 and writes one data word to stack cache 155. Themost common stack manipulation for a stack-based computing system is topop the top two data words off of the stack and to push a data word ontothe top of the stack. Therefore, the circuit of FIG. 11 is configured tobe able to provide read pointers to the value of pointer OPTOP and thevalue of pointer OPTOP modulo-SCS minus one, and a write pointer to thecurrent value of OPTOP modulo-SCS minus one.

Multiplexer (MUX) 1110 drives a read pointer RP1 for read port 640. Aselect line RS1 controlled by hardware processor 100 determines whethermultiplexer 1110 drives the same value as pointer OPTOP or a readaddress R₋₋ ADDR1 as provided by hardware processor 100.

Multiplexer 1120 provides a read pointer RP2 for read port 650. Moduloadder 1140 modulo-SCS adds negative one to the value of pointer OPTOPand drives the resulting sum to multiplexer 1120. A select line RS2controlled by hardware processor 100 determines whether multiplexer 1120drives the value from modulo adder 1140 or a read address R₋₋ ADDR2 asprovided by hardware processor 100.

Multiplexer 1130 provides a write pointer WP for write port 630. Amodulo adder 1150 modulo-SCS adds one to the value of pointer OPTOP anddrives the resulting sum to multiplexer 1130. Select lines WS controlledby hardware processor 100 determines whether multiplexer 1130 drives thevalue from modulo-SCS adder 1140, the value from modulo-SCS adder 1150,or a write address W₋₋ ADDR as provided by hardware processor 100.

FIG. 12 shows a circuit that generates a read pointer R for read port640 or read port 650 in embodiments allowing accessing stack cachememory circuit using pointer VARS. Multiplexer 1260 drives read pointerR to one of several input values received on input ports 1261-1267 asdetermined by selection signals RS. Selection signals RS are controlledby hardware processor 100. The value of pointer OPTOP is driven to inputport 1261. Modulo-SCS adder 1210 drives the modulo-SCS sum of the valueof pointer OPTOP with negative one to input port 1262. Modulo-SCS adder1210 drives the modulo-SCS sum of the value of pointer OPTOP withnegative two to input port 1263. The value of pointer VARS is driven toinput port 1264. Modulo-SCS adder 1230 drives the modulo-SCS sum of thevalue of pointer VARS with one to input port 1265. Modulo-SCS adder 1240drives the modulo-SCS sum of the value of pointer VARS with two to inputport 1266. Modulo adder-SCS 1250 drives the modulo-SCS sum of the valueof pointer VARS with three to input port 1263. Other embodiments mayprovide other values to the input ports of multiplexer 1260.

Thus by using the stack cache according to the principles of theinvention, a dribbling management unit can efficiently control transfersbetween the stack cache and the stack. Specifically, the dribblingmanagement unit is able to transfer data out of the stack cache to makeroom for additional data as necessary and transfer data into the stackcache as room becomes available transparently to the stack-basedcomputing system using the stack management unit.

The various embodiments of the structure and method of this inventionthat are described above are illustrative only of the principles of thisinvention and are not intended to limit the scope of the invention tothe particular embodiments described. In view of this disclosure, thoseskilled-in-the-art can define other memory circuits, registers,counters, stack-based computing systems, dribble management units, fillcontrol units, spill control units, read ports, write ports, and usethese alternative features to create a method or system of stack cachingaccording to the principles of this invention. ##SPC1##

We claim:
 1. In a processor having a stack, a stack cache managementunit for caching said stack comprising:a stack cache having a stackcache memory circuit coupled to said stack, said stack cache memorycircuit having a plurality of memory locations; a cache bottom pointerpointing to and defining a bottom memory location within said stackcache memory circuit; a spill control unit coupled to transfer a firstdata word stored in said bottom memory location from said stack cache tosaid stack; and a fill control unit coupled to transfer a second dataword from said stack to said bottom memory location or a memory locationadjacent said bottom memory location.
 2. The stack cache management unitof claim 1, wherein said stack cache further comprises:a first read portcoupled between said stack cache memory circuit and said stack, whereinsaid spill control unit controls said first read port; and a first writeport coupled between said stack cache memory circuit and said stack,wherein said fill control unit controls said first write port.
 3. Thestack cache management unit of claim 2, further comprising an optoppointer pointing to and defining a top memory location of said stackcache memory circuit.
 4. The stack cache management unit of claim 3,further comprising a stack cache status circuit coupled to receive saidoptop pointer and said cache bottom pointer, said stack cache statuscircuit configured to calculate a number of free memory locations insaid stack cache memory circuit.
 5. The stack cache management unit ofclaim 4, wherein said spill control unit transfers said first data wordonly if said number of free memory locations is less than a cache highthreshold.
 6. The stack cache management unit of claim 5, wherein saidspill control unit comprises:a programmable register containing saidcache high threshold; and a comparator coupled to said stack cachestatus circuit and said programmable register.
 7. The stack cachemanagement unit of claim 4, wherein said fill control unit transferssaid second data word only if said number of free memory locations isgreater than a cache low threshold.
 8. The stack cache management unitof claim 7, wherein said fill control unit comprises:a programmableregister containing said cache low threshold; and a comparator coupledto said stack cache status circuit and said programmable register. 9.The stack cache management unit of claim 3, wherein said spill controlunit transfers said first data word only if said optop pointer isgreater than a high water mark.
 10. The stack cache management unit ofclaim 9, wherein said spill control unit comprises:a programmableregister containing said high water mark; and a comparator coupled tosaid stack cache status circuit and said programmable register.
 11. Thestack cache management unit of claim 3, wherein said fill control unittransfers said second data word only if said optop pointer is less thana low water mark.
 12. The stack cache management unit of claim 11,wherein said spill control unit comprises:a programmable registerconfigured to contain said low water mark; and a comparator coupled tosaid stack cache status circuit and said programmable register.
 13. Thestack cache management unit of claim 1, wherein said stack cache memorycircuit is organized as a circular buffer.
 14. The stack cachemanagement unit of claim 1, wherein said first data word and said seconddata word are the same data word.
 15. In a processor having a stack, astack cache management unit for caching said stack comprising:a stackcache having a stack cache memory circuit coupled to said stack, saidstack cache memory circuit having a plurality of memory locations; acache bottom pointer pointing to and defining a bottom memory locationwithin said stack cache memory circuit; a spill control unit coupled totransfer a first data word stored in said bottom memory location fromsaid stack cache to said stack; a fill control unit coupled to transfera second data word from said stack to said bottom memory location or amemory location adjacent said bottom memory location; a first read portcoupled between said stack cache memory circuit and said stack, whereinsaid spill control unit controls said first read port; a first writeport coupled between said stack cache memory circuit and said stack,wherein said fill control unit controls said first write port; a secondread port coupled to said stack cache memory circuit; a third read portcoupled to said stack cache memory circuit; and a second write portcoupled to said stack cache memory circuit; and a stack control unitconfigured to control said second read port, said third read port andsaid second write port.
 16. The stack cache management unit of claim 15wherein said stack is for a JAVA virtual machine.
 17. The stack cachemanagement unit of claim 15, wherein each of said memory locationscomprises:a valid bit; a saved bit; and a data section.
 18. The stackcache management unit of claim 17, wherein said spill control unit setssaid saved bit of said bottom memory location to a saved logic state ifsaid first data word is transferred to said stack.
 19. The stack cachemanagement unit of claim 17, wherein said fill control unit sets saidvalid bit of said memory location preceding said bottom memory locationif said second data word is transferred from said stack.
 20. In aprocessor, a method for caching a stack in a stack cache having aplurality of memory locations, an optop pointer pointed at a top memorylocation of said stack cache, and a bottom pointer pointed at a bottommemory location of said stack cache, said method comprising:writing anew data word for said stack at said optop memory location; incrementingsaid optop pointer; spilling a first data word from said stack cache tosaid stack if a spill condition exists; and filling a second data wordfrom said stack to said stack cache if a fill condition exists whereinsaid filling a second data word from said stack to said stack cachecomprises:decrementing said bottom pointer; and transferring said seconddata word from said stack to said bottom memory location, or to a memorylocation preceding said bottom memory location.
 21. The method of claim20, further comprising:reading a stacked data word from said stack cacheat said top memory location; and decrementing said optop pointer.
 22. Amethod for caching a stack in a stack cache having a plurality of memorylocations, an optop pointer pointed at a top memory location of saidstack cache, and a bottom pointer pointed at a bottom memory location ofsaid stack cache, said method comprising:writing a new data word forsaid stack at said optop memory location; incrementing said optoppointer; spilling a first data word from said stack cache to said stackif a spill condition exists; filling a second data word from said stackto said stack cache if a fill condition exists; reading a first stackeddata word from said stack cache at said top memory location; and readinga second stacked data word from said stack cache at a memory locationpreceding said top memory location; and decrementing said optop pointerby two.
 23. A method for caching a stack in a stack cache having aplurality of memory locations, an optop pointer pointed at a top memorylocation of said stack cache, and a bottom pointer pointed at a bottommemory location of said stack cache, said method comprising:writing anew data word for said stack at said optop memory location; incrementingsaid optop pointer; spilling a first data word from said stack cache tosaid stack if a spill condition exists; and filling a second data wordfrom said stack to said stack cache if a fill condition exists and;determining if said spill condition exists wherein said determining ifsaid spill condition exists comprises:calculating a number of freememory locations; and comparing said number of free memory locations toa high cache threshold.
 24. A method for caching a stack in a stackcache having a plurality of memory locations, an optop pointer pointedat a top memory location of said stack cache, and a bottom pointerpointed at a bottom memory location of said stack cache, said methodcomprising:writing a new data word for said stack at said optop memorylocation; incrementing said optop pointer; spilling a first data wordfrom said stack cache to said stack if a spill condition exists; fillinga second data word from said stack to said stack cache if a fillcondition exists; determining if said spill condition exists whereinsaid determining if said spill condition exists comprises:comparing saidoptop pointer to a high water mark; incrementing said high water mark ifsaid spill condition exists; and decrementing said high water mark ifsaid fill condition exists; determining if said fill condition existswherein said determining if said fill condition existscomprises:comparing said optop pointer to a low water mark; incrementingsaid low water mark if said spill condition exists; and decrementingsaid low water mark if said fill condition exists.
 25. A method forcaching a stack in a stack cache having a plurality of memory locations,an optop pointer pointed at a top memory location of said stack cache,and a bottom pointer pointed at a bottom memory location of said stackcache, said method comprising:writing a new data word for said stack atsaid optop memory location; incrementing said optop pointer; spilling afirst data word from said stack cache to said stack if a spill conditionexists; and filling a second data word from said stack to said stackcache if a fill condition exists and; determining if said fill conditionexists wherein said determining if said fill condition existsincludes:calculating a number of used memory locations; and comparingsaid number of used memory locations to a low cache threshold.
 26. Amethod for caching a stack in a stack cache having a plurality of memorylocations, an optop pointer pointed at a top memory location of saidstack cache, and a bottom pointer pointed at a bottom memory location ofsaid stack cache, said method comprising:writing a new data word forsaid stack at said optop memory location; incrementing said optoppointer; spilling a first data word from said stack cache to said stackif a spill condition exists; and filling a second data word from saidstack to said stack cache if a fill condition exists and; determining ifsaid fill condition exists wherein said determining if said fillcondition exists comprises:comparing said optop pointer to a low watermark; incrementing said low water mark if said spill condition exists;and decrementing said low water mark if said fill condition exists.